Terminal double bond distribution

We will introduce this approach in the case of the 2D CLD/DBD computation for mixed-metallocene polymerization of ethylene. Subsequently, we present applications of the approach to the 3D problems of radical polymerization of vinyl acetate (CLD/DBD/number of terminal double bonds distribution), AB radical copolymerization (CLD/comonomer composition distribution/sequence length distribution), and finally the 2D problem of radical polymerization of polyethylene, where random scission is a complicating factor. 

Propene- and butene-oligomers are complex mixtures. A typical isomer distribution is shown in Fig. 24. According to the thermodynamical stability the double bonds are distributed along the chain, terminal double bonds are present only in traces. To get predominant terminal products, a catalyst must provide extremely fast terminal hydroformylation activity for the traces of terminal olefins, a high isomerization activity to supply the terminal double bonds as fast as they are consumed, and low hydroformylation activity for internal double bonds. 

Fig. 14 Accumulated weight fraction distribution development with and without terminal double bond polymerization...

Nonlinear polymer formation in emulsion polymerization is a challenging topic. Reaction mechanisms that form long-chain branching in free-radical polymerizations include chain transfer to the polymer and terminal double bond polymerization. Polymerization reactions that involve multifunctional monomers such as vinyl/divinyl copolymerization reactions are discussed separately in Sect. 4.2.2. For simplicity, in this section we assume that both the radicals and the polymer molecules that formed are distributed homogeneously inside the polymer particle. 

The development takes into account transfer to monomer, transfer to polymer, and terminal double bond polymerization. For the vinyl acetate system where transfer to monomer is high, the generation of radicals by transfer to monomer is much greater than the generation of radicals by initiation, so that essentially all radicals present have terminal double bonds hence, effectively all dead polymer molecules also have a terminal double bond. Thus, for vinyl acetate polymerization, the terminal double bond polymerization can be significant, and has been built into the development. The equations for the moments of the molecular weight distribution and the average number of branches per polymer molecule are as follows ... 

The structure written for I is satisfactory for olefins which can have only internal or only terminal double bonds (ethylene, propylene, cyclohexene). We find, however, that although internal olefins are thermodynamically more stable than terminal olefins under reaction conditions, the products obtained in the hydroformylation reactions are largely derived by addition to the terminal carbons. For example, the distribution of alcohols secured from 1-pentene and 2-pentene is about the same 13, 14), 50-55% of n-hexanol, 35-40% of 2-methylpentanol-l, and 10% of 2-ethylbutanol-l. In each case the chief product can be obtained only by the addition of the formyl group to the No. 1 carbon atom. If we assume that hydroformylation occurs only at the double bond, we may ask how it is possible to form a straight-chain aldehyde from an internal olefin. 

The product distribution will thermodynamically favor the "1,4" adduct over the "1,2" adduct. This is because an internal double bond ("1,4" adduct) is more stable than a terminal double bond ("1,2" adduct) that is, the more substituted an alkene, the more stable the molecule. Hence, the product distribution depends totally upon the reaction conditions. 

The findings of other authors that straight chain olefins, with both terminal and non-terminal double bonds, always form the same ratio of isomeric aldehydes can be explained by the fact that they limited their experiments to reaction temperatures of 140 to 170 C. In this range, indeed, nearly the same isomer distributions are obtained from both types of olefins, as can be seen from fig. 7. 

CLD/Number of Terminal Double Bonds (TDB) Distribution for Poly(vinyl acetate) -More than one TDB per Chain... 

J. B. P. Soares and A. E Hamielec, Bivariate chain length and long chain branching distribution for copolymerization of olefins and polyolefin chains containing terminal double-bonds. Macromol Theory Simul. (1996) 5, pp. 547-572... 

Addition polymers, which are also known as chain growth polymers, make up the bulk of polymers that we encounter in everyday life. This class includes polyethylene, polypropylene, polystyrene, and polyvinyl chloride. Addition polymers are created by the sequential addition of monomers to an active site, as shown schematically in Fig. 1.7 for polyethylene. In this example, an unpaired electron, which forms the active site at the growing end of the chain, attacks the double bond of an adjacent ethylene monomer. The ethylene unit is added to the end of the chain and a free radical is regenerated. Under the right conditions, chain extension will proceed via hundreds of such steps until the supply of monomers is exhausted, the free radical is transferred to another chain, or the active site is quenched. The products of addition polymerization can have a wide range of molecular weights, the distribution of which depends on the relative rates of chain grcnvth, chain transfer, and chain termination. 

Recently, Kondo and coworkers reported on the polymerization of St with diphenyl diselenides (37) as the photoiniferters (Eq. 39) [ 162]. In the photopolymerization of St in the presence of 37a and 37b, the polymer yield and the molecular weight of the polymers increased with reaction time. The chain-end structure of the resulting polymer 38 was characterized. Polymer 38 underwent the reductive elimination of terminal seleno groups by reaction with tri-n-butyltin hydride in the presence of AIBN (Eq. 40). It also afforded the poly(St) with double bonds at both chain ends when it was treated with hydrogen peroxide (Eq. 41). They also reported the polymerization of St with diphenyl ditelluride to afford well-controlled molecular weight and its distribution [163]. 

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